Exhaust Gas Treatment

ABSTRACT

A device  1 , capable of being placed in-line in the exhaust conduit of an IC engine upstream of an SCR catalyst, which produces a gaseous hydrolysis product, containing ammonia, which is added to the exhaust gas in a controlled manner to pass therewith through the SCR catalyst to reduce the NOx content of the exhaust gas. The device  1  has an inlet  2  and an outlet  3  for the exhaust gas flowing therethrough and comprises an outer body  4 , which forms a pressure barrier, and passing through the outer body  4  is an inner body  5  which comprises a flowpath longitudinally therethrough from the inlet  2  to the outlet  3 . The device  1  is split into two sections, a reactor section with an inlet for pressurised aqueous urea and comprising an upper and lower area connected by a plurality of tubes  10  which pass through the exhaust gas for heat exchange and wherein the hydrolysis of urea occurs at elevated temperature and pressure and a reservoir section connected to the reactor section via a pressure release valve to allow the gaseous hydrolysis product to pass from the reactor to the reservoir. The device further comprises a valve  17  for dosing the gaseous hydrolysis product into the exhaust gas.

The present invention relates to an apparatus for reducing emissions ofNitrogen oxides (NOx) in exhaust gasses of an internal combustion (IC)engine.

The introduction of reagents into the flow of an exhaust gas of an ICengine prior to the gas passing through a catalyst in order to effectselective catalytic reduction (SCR) of NOx is well known.

The known systems principally fall into one of two categories, thosewhich introduce gaseous ammonia into the exhaust conduit and those whichintroduce into the exhaust conduit a liquid reagent which decomposesinto ammonia gas in the conduit.

The introduction of gaseous ammonia into exhaust gasses for SCR purposeshas been known for a long time in association with static systems, forexample the after-treatment of flue gas in power plants. Over time, thebenefit of SCR has been realised in mobile solutions, initially in theshipping industry and more recently in the motor vehicle industry. Wherethe application is mobile, for example a motor vehicle, there are,however, safety implications in carrying a sufficiently large supply ofammonia on board to cope with requirements over an acceptable period oftime. For example a rupture of the ammonia vessel, for example in acrash, could cause the release of large volumes of ammonia into theatmosphere. In addition there are additional risks of ammonia releasewhen handling and refilling the ammonia vessel, for example at roadsideservice stations.

One solution to this problem has been to inject a liquid reagent intothe hot exhaust gas where it decomposes into ammonia. The liquid reagentis, at ambient temperatures, a stable medium, but it decomposes atelevated temperatures to form at least ammonia gas. It is preferably anaqueous solution of urea or related substance such as biuret or ammoniumcarbamate, collectively referred to, and defined, herein as “urea”.While this solution to the problem provides a satisfactory result, thereare a number of problems associated with it. Firstly, the liquid isinjected through a nozzle as a fine spray of droplets into the fastflowing exhaust gas in which it preferably fully decomposes into atleast ammonia gas prior to contacting the SCR catalyst. As this is notan instantaneous process, there needs to be a minimum separationdistance between the injector and the SCR catalyst to allow sufficienttime to allow the full decomposition of the liquid into gas prior to itcontacting the SCR catalyst. Secondly is the problem of precipitation ofsolids from the urea solution throughout the system and especially inthe injector nozzle and catalyst. Solid formation in the nozzles tendsto occur particularly where dormant urea solution has resided at a hightemperature under minimal pressure for a period of time in the injectornozzle. The solids may frequently block the nozzles, calling for complexcontrol systems either to purge the nozzle, e.g. with pressurised air,or to re-circulate the urea so that it does not have the requisite timeat elevated temperature for the precipitation to occur. Solidificationof solids on the catalyst which occurs particularly when the liquid isdosed at low temperatures below about 180 degrees C. reduces theefficiency of the catalyst and increases the back pressure the catalystcreates within the exhaust system and therefore in time the catalystwill need replacing.

An alternative solution to the problem has been proposed in U.S. Pat.No. 6,361,754 and comprises hydrolysing aqueous urea under pressure at ahigh temperature so that it decomposes into at least gaseous ammonia andthen introducing the gaseous ammonia into the exhaust conduit. Whilethis is an efficient method of preparing ammonia gas in situ, as theheating is dependant on the reactor being placed in the exhaust conduitand the pressure under which the urea is being maintained will varydepending on the dosing of the gas into the exhaust, it is very hard tomaintain a stable reaction and ammonia concentration within thehydrolysis gas will vary. Also, all components of the system, of whichthere are many, need to be maintained at a minimum temperature andpressure to prevent the precipitation of solids. The operationalpressure of the system is directly linked to the dosing and ifcompensated by continual supply of aqueous urea, to maintain constantammonia concentration in the hydrolysis gas, then in times of peakdemand the aqueous urea may pass fully through the reactor and be doseddirectly into the exhaust.

U.S. Pat. No. 6,399,034 discloses an alternative solution which utilisesdecomposition of an alternative reagent, for example an aqueous solutionof ammonium carbamate, which decomposes at a lower temperature, andstores the ammonia gas produced in an intermediate storage vessel anddoses the gas from that vessel into the exhaust. The aqueous solution isheated by the engine cooling system which is capable of providing thelower heat requirements to decompose ammonium carbamate, but which wouldnot be sufficient to hydrolyse urea at the rate required for NOxreduction in the exhaust of an IC engine.

Both of the above solutions and similar solutions used in staticapplications in the power industry are complex assemblies containing alarge number of parts and interconnecting tubing which firstly need tobe maintained at an appropriate temperature to prevent precipitation ofsolids from the liquid phase and deposition of solids from the gaseousphase, and which secondly are hard to retrofit to existing vehicles.This is commonly done by thermally lagging all the interconnectingtubing and by heating the interconnecting tubing, which while suitablefor static systems running constantly under steady state conditions, isnot suitable for commercial vehicles which operate under a stop/startregime. In particular, at start-up, until enough exhaust gas has passedthrough the exhaust conduit in order to heat it to the requiredtemperature, there is a danger of depositions of solids creatingblockages in the conduit and causing the gas pressure in it to rise todangerous levels.

A further problem with existing systems is that, because of the numberof parts, retrofitting them to existing vehicles is a complex procedure.

It is the purpose of the present invention to mitigate some of the aboveproblems by providing a simplified apparatus for the production ofammonia gas for use in SCR systems of IC engines, especially but notexclusively diesel engines.

According to a first aspect of the present invention there is provided aunitary device for generating and feeding gaseous hydrolysis productcomprising ammonia, formed by the hydrolysis of an aqueous solution ofurea (as hereinbefore defined) at elevated temperature and pressure,into the exhaust gas of an IC engine as it flows through the exhaustsystem of the engine, the device being adapted to be placed in theexhaust system so that the exhaust gas will flow through it during use,and comprising

a) a housing having an inlet for the exhaust gas and an outlet for theexhaust gas;b) a reaction vessel located at least partially within the housingbetween the inlet and the outlet for containing an aqueous solution ofurea and arranged such that, in use, the vessel and therefore the ureasolution become heated by means of heat exchange with the exhaust gas asit flows from the inlet to the outlet;c) a urea solution inlet to the reaction vessel and a gaseous hydrolysisproduct outlet from the reaction vessel;d) a reservoir for receiving and storing gaseous hydrolysis product;e) a valve in the outlet from the reaction vessel and adapted to permitthe contents of the reaction vessel, in use, to attain an elevatedpressure as it becomes heated, and to discharge gaseous hydrolysisproduct into the reservoir; andf) a conduit for interconnecting the reservoir and the exhaust system,the conduit including a valve to selectively control the feed ofhydrolysis product stored in the reservoir into the exhaust gas via theconduit.

Preferably the valves are placed at least partially outside the housingsuch that they are at least partially protected from direct exposure tothe hot exhaust gasses.

By having substantially all the components within the same unit, heattransfer from the exhaust gas by conduction quickly heats the componentsupon start up and maintains them at an elevated temperature preventingammonium carbamate and other solids from solidifying from the gaseoushydrolysis product thereby preventing potential blockages of the systemand maintaining a system which can operate safely whilst producing gasunder pressure. Where the valves are partially placed outside of theunit the parts of the valve through which the hydrolysis product flowsis maintained at an elevated temperature sufficient that the hydrolysisproduct stays in gaseous form as it passes therethrough.

As the device, in its preferred form, can be supplied as essentially oneunit it is simple to fit both for new builds and as a retrofit toexisting vehicles as it simply replaces a section of the current exhaustconduit.

The valve (e) in the outlet from the reaction vessel is adapted topermit the contents of the reaction vessel, in use, to attain anelevated pressure as it becomes heated. The valve may take a number offorms. In one preferred arrangement the valve (e) actuates in responseto the pressure within the reaction vessel and preferably periodicallydischarges gaseous hydrolysis product into the reservoir. This can be anactive actuation where the pressure is measured in the reaction vesseland the valve is actuated via a control system depending on the signalreceived from a pressure transducer situated in the reaction vessel.Alternatively this can be a passive actuation where the valve is selfactuating when a preset pressure occurs on its inlet side, i.e. it is asimple mechanical back pressure valve. By maintaining a substantiallyconstant pressure within the reactor vessel the concentration of theammonia within the hydrolysis gas remains substantially constant.

In an alternative preferred arrangement the valve actuates in responseto the temperature of the aqueous solution of urea. This is preferablydone by measuring the temperature within the aqueous urea solution andactuating the valve in response to the measured temperature. As thereaction occurs within the reaction vessel and the pressure rises thetemperature within the solution can also be raised until both areelevated, and as there is a direct relationship between the two, controlof the release of the gaseous hydrolysis product can be based on either.The valve for controlling the release of the gaseous hydrolysis productis preferably placed in a bulkhead between the reaction vessel and thereservoir.

In a preferred arrangement of the present invention the system furthercomprises a sensor placed within the exhaust gas flow to measure thequantity of NOx therein. The NOx sensor may be upstream of the point ofintroduction of the ammonia containing hydrolysis gas or downstream ofthe SCR catalyst and would either measure the NOx output of the engineor the NOx output of the vehicle respectively. If the NOx output of theengine is measured then the signal is used to predict the requiredvolume of the gaseous hydrolysis product required to be dosed into thegas to effect its removal (i.e. open loop control), whereas if the NOxoutput of the vehicle is sensed then more or less gaseous hydrolysisproduct will be dosed into the exhaust gas depending whether the sensedNOx level is above or below a target level (i.e. closed loop control).In an alternative arrangement an ammonia sensor is placed downstream ofthe SCR catalyst to measure ammonia slip (the amount of ammonia passingun-reacted through the SCR catalyst). The control system can then senseif too much ammonia containing hydrolysis gas is being added to theexhaust flow and reduce the amount accordingly.

Preferably the device is close coupled to an SCR catalyst, or optionallyis contained within one and the same unitary housing with an SCRcatalyst connectable in line in the exhaust system. Preferably thedownstream end of the SCR catalyst is coated with a catalyst thatconverts any un-reacted ammonia in the exhaust gas so that ammonia isnot released into the environment.

Preferably the exhaust gas flowpath between the point of introduction ofthe hydrolysis gas and the SCR catalyst is shaped to induce mixing ofthe hydrolysis gas with the exhaust gas. Preferably the ammonia gas isintroduced substantially on the axis of the exhaust gas flow path and isintroduced in a direction substantially perpendicular to the directionof the flow. Preferably a number of radially spaced inlets are situatedadjacent one another substantially perpendicular the flow. Preferablythe point of introduction is substantially at the mouth of a truncatedconical section of the flowpath and the flow of exhaust gas andhydrolysis gas into the cone induces mixing. Preferably the flowpathbetween the point of introduction of the hydrolysis gas and the SCRcatalyst has at least one substantially 90 degree bend causingturbulence in the flowpath further inducing mixing. Preferably theexhaust gas and hydrolysis gas enter a substantially cylindrical vortexchamber, upstream of the SCR catalyst, substantially perpendicularly tothe radius of the chamber and exits the chamber along its central axis,the vortex within the chamber further inducing mixing of exhaust gas andhydrolysis gas.

Preferably, contained within the same unit as the device, is anoxidation catalyst through which the exhaust gas flows prior to theaddition of the hydrolysis gas, Preferably the oxidation catalyst issized to oxidise a proportion of the Nitric Oxide in the exhaust gasthat a favourable mixture, preferably approximately 50/50, of NO and NO₂is achieved in the exhaust gas. Preferably the oxidation catalyst,device and SCR are all contained within one unit having an exhaust inletand an exhaust outlet and connectible in line in the exhaust system of avehicle.

Preferably, downstream of the oxidation catalyst is a diesel particulatefilter. Preferably the diesel particulate filter is contained in one andthe same unit as the oxidation catalyst, device and SCR catalyst.

By containing the device within the same housing as the catalysts andoptionally a filter the entire exhaust treatment system, comprising thefunctions of removing diesel particulates, preparing the exhaust for NOxtreatment, adding an appropriate reagent to the exhaust gas and thenpassing the admixture through or over a catalyst to reduce the NOxcontent of the exhaust gas can be performed by one unit which may besupplied to the vehicle manufacturer as a unit ready for incorporationinto the vehicle. Preferably the aforementioned NOx sensor is alsocontained within the same unitary housing.

Preferably the reservoir is provided with a heater to increase itstemperature.

In one arrangement of the invention the device comprises an outer body,which forms a pressure barrier, and passing through the outer body is aninner body which comprises a flowpath longitudinally therethrough withan inlet and an outlet for the exhaust gas, the outer and inner bodiesforming two chambers therebetween. In a preferred arrangement, one ofthe chambers is situated substantially above the inner body and theother is situated substantially below the inner body. The two chambersare connected by at least one fluid passageway, the two chambers and theat least one fluid passageway comprising the reaction vessel.

The fluid passageway(s) and optionally at least a section of the wallsof the two chambers are, in use, in thermal contact with the exhaustgas.

Preferably the fluid passageway(s) between the two chambers passesthrough the exhaust gas flowpath formed by the inner body and preferablycomprises a number of tubes. Alternatively the fluid passageway(s) maypass around the sides of the inner body.

Preferably, the inner and outer bodies extend beyond the reaction vesseland the volume defined between said inner and outer bodies in theirextended sections is separated from the reaction vessel by a bulkhead.The extended section of the inner and outer sections is enclosed on theother end to form an enclosed reservoir area abutting the reactionvessel and through which the inner body passes.

In this arrangement the device is optionally further provided with aby-pass valve to selectively bypass a proportion of the exhaust gas sothat it does not directly heat the fluid passageways of the reactionvessel whereby the heat input to the reaction vessel can be varied. Thisenables the output of the reaction vessel to be controlled as the demandfluctuates. Preferably the inner body comprises two exhaust gasflowpaths, only one of which is in thermal contact with the fluidpassageways of the reaction vessel and the by-pass valve controls theproportion of the exhaust gas flow which passes through each exhaust gasflowpath.

In a second arrangement the device comprises, in part, a rear sectioncomprising two substantially cylindrical upright tubes and an enclosedcavity therebetween. The two upright tubes contain the reaction vesseland reservoir respectively. The tube containing the reaction vessel hasan inlet for the exhaust gas and is in fluid communication with theenclosed cavity. The hot exhaust gasses enter flow through the tubepassing over the reaction vessel and heat it. The outer surface of thetube containing the reservoir is partially in direct fluid contact withthe hot exhaust gasses but the reservoir itself is insulated from thedirect heat, preferably by an air gap. Heat transfer through the wall ofthe tube containing the reservoir and across the air gap is sufficientto maintain the reservoir at a sufficiently high temperature to preventcondensation of the hydrolysis gas or solidification of salts out of thehydrolysis gas during operation. The enclosed cavity has an openingtherein for the exhaust gas to pass through prior to entering a dieselparticulate filter and/or an oxidation catalyst.

Preferably attached to the exterior of the rear section via a framework,are the catalysts and mixing elements and optionally the dieselparticulate filter. An outer casing fits over these components forming atreatment enclosure. Preferably an outlet from the unit passes from thetreatment enclosure through the enclosed cavity in the rear section toallow the exhaust to exit the unit for eventual discharge.

Preferably the device is mounted on a commercial vehicle such that therear section is closest to the centre of the vehicle and the treatmentenclosure extends outwards therefrom such that, in event of a collision,the treatment enclosure and components therein form a sacrificial‘crumple zone’ to absorb the energy of impact and protect thepressurised reaction vessel and reservoir from direct impact

Preferably the top of the reservoir and reaction vessel abut a manifoldplate, said manifold plate providing a barrier between the hot areabelow it (the two tubes) and the cooler area above it. Preferably allthe valves and sensors are placed in, or pass through, the cool areasuch that their electronics and some other function critical parts canbe protected from direct exposure to the hot environment. Preferably themanifold plate includes a heat shield between the hot area and the coolarea. Preferably the valves and sensors have a cover sealed thereoversuch that the exterior of the device can be washed down withoutaffecting the electronics. Preferably the covers are of a thermallyconductive material and include a number of cooling fins to assist inremoving any heat from this area. Preferably the covers are made ofaluminium.

In use the system is attached via a urea inlet line to a urea pump and aurea solution tank. Preferably the urea tank is provided with a qualitysensor to detect the quality of urea and create an alert or render thedevice inoperable if the urea is not of the correct quality, for exampleif it does not have the correct percentage of urea. Equally the sensorwill be able to detect if a different substance, e.g. water or dieselhas been deliberately or inadvertently put into the urea tank. In onearrangement the pump is integral with the urea tank. Preferably the ureatank and urea line between the urea tank and the reaction vessel inletis heated to prevent urea freezing in the urea line.

According to a second aspect of the invention there is provided ahydrolysis gas reservoir for receiving ammonia containing gas from ahydrolysis reaction vessel, the reservoir comprising a body and an uppermanifold, said upper manifold having passageways therein to accommodatevarious sensors and at least one valve and having heating meansassociated therewith to maintain said upper manifold at a substantiallyconstant temperature, thereby preventing blockages of the passagewaystherein by deposition of solid ammonia salts formed at lowertemperatures.

Preferably the heating means comprises electric heating elements, morepreferably the heating means comprises a plurality of elongate cartridgeheaters inserted substantially radially into the upper manifold.

Preferably the heating means maintain the upper manifold at atemperature in the range 100 to 300 degrees centigrade. More preferablythe heating means maintain the upper manifold at a temperature in therange 180 to 220 degrees centigrade, ideally 200 degrees.

Preferably attached to a passageway of the upper manifold is a pressurerelief valve that releases the hydrolysis product from the reservoirshould the pressure therein exceed a certain value. Preferably any gasbeing released therefrom is released into a small reservoir of water tocondense the ammonia and prevent it being released directly into theatmosphere. Alternatively the gas may be vented directly into theexhaust gas stream, ideally prior to the oxidation catalyst.

In one arrangement, attached to a passageway of the upper manifold,there is preferably a dosing valve for dosing the hydrolysis gas intothe exhaust gas stream.

In an alternative preferred arrangement the manifold has a valve sattherein between two of the passageways, one leading from the interior ofthe reservoir and forming a valve inlet and the other exiting the sideof the manifold forming an outlet. Preferably a valve actuator andassociated valve armature are connectable to the manifold, the valveactuator operable to move the valve armature on and off the valve seatthereby preventing or allowing flow.

Preferably located in passageways into the upper manifold are atemperature sensor and/or a pressure sensor.

Preferably the upper manifold is welded to the body. Preferably theupper manifold has means for attaching it to a manifold plate accordingto the first aspect of the invention such that the valves and sensorsprotrude through the manifold plate into the cool area above it.Preferably said means for attaching the upper manifold to the manifoldplate comprise a plurality of flanges adapted to take a screw or bolt.

According to a third aspect of the present invention there is provided adevice for generating gaseous hydrolysis product comprising ammonia,formed by the hydrolysis of an aqueous solution of urea (as hereinbeforedefined) at elevated temperature and pressure, the device being adaptedto be placed in the exhaust system so that the exhaust gas will flowthrough it during use, and comprising

-   a) a first substantially upright and cylindrical tube enclosed at    its upper end and open at its lower end and having an inlet and an    outlet on its sides;-   b) an elongate reaction vessel located in the tube for containing an    aqueous solution of urea and arranged such that, in use, the vessel    and therefore the urea solution become heated by means of heat    exchange with the exhaust gas as it flows from the inlet to the    outlet; and-   c) a urea solution inlet to the reaction vessel and a gaseous    hydrolysis product outlet from the reaction vessel;    wherein said reaction vessel is attached to the upper enclosed end    of the first tube and sealingly engages with the first tube at its    lower end preventing the exhaust gas from escaping out of the open    lower end of the first tube.

As the reaction vessel is elongate its primary direction of thermalexpansion and contraction will be along its axis. As the reaction vesselis only attached by one end it is free to expand within the first tubeas it heats up and contract as it cools down. In addition, in the caseof a rupture in the vessel anywhere apart from the bottom, as tube isenclosed at its upper end the expansion of the gas as it exits theruptured reaction vessel will tend to force the reservoir out of theopen bottom end of tube.

Preferably the reaction vessel or the relief valve is provided with astructurally weak point in its upper end of the reaction vessel/reliefvalve assembly that will rupture at a lower pressure than the rest ofthe reaction vessel ensuring that in the case of excessive pressurebuild up in the reaction vessel the structurally weak point will ruptureand the gas in the reaction vessel will expand therethrough forcing thereaction vessel downwards and enhancing the effect of the downwardsprojection of the reaction vessel due to the restraint of the tube.

In one preferred arrangement the reaction vessel has a circumferentialseal attached to the outer surface of its lower end and the said sealslides in the tube as the reaction vessel expands and contracts. In analternative preferred arrangement the tube has a circumferential sealattached to the inner surface of its lower end and the reaction vesselslides past the seal as it expands and contracts.

Preferably the device further comprises a second substantially uprightand cylindrical tube having an enclosed upper end and an open lower end,said second tube housing a substantially elongate reservoir to collectthe gaseous hydrolysis product produced in the reaction vessel and saidreservoir attached to the upper enclosed end of the tube and sealinglyengaging with the tube at its lower end.

The reservoir is able to expand and contract in a similar way as thereaction vessel.

Preferably the exterior of the second tube is at least partially heatedby the hot exhaust gasses.

Preferably the reservoir is provided with a structurally weak point inits upper end that will rupture at a lower pressure than the rest of thereservoir ensuring that in the case of excessive pressure build up inthe reservoir the structurally weak point will rupture and the gas inthe reservoir will expand therethrough forcing the reservoir downwardsand enhancing the effect of its downwards projection due to therestraint of the tube.

In one preferred arrangement the reservoir has a circumferential sealattached to the outer surface of its lower end and the said seal slidesin the tube as the reservoir expands and contracts. In an alternativepreferred arrangement the tube has a circumferential seal attached tothe inner surface of its lower end and reservoir slides past the seal asit expands and contracts.

Preferably the first and second substantially upright tubes form the twosubstantially upright tubes of the rear section of the secondarrangement of the first embodiment of the invention.

According to a fourth aspect of the present invention there is provideda device for generating gaseous hydrolysis product comprising ammonia,formed by the hydrolysis of an aqueous solution of urea (as hereinbeforedefined) at elevated temperature and pressure, for feeding into theexhaust gas of an IC engine as it flows through the exhaust system ofthe engine, the device being adapted to be placed in the exhaust systemso that the exhaust gas will flow through it during use, and comprising

a) a housing having an inlet for the exhaust gas and an outlet forthexhaust gas;b) a reaction vessel located at least partially within the housingbetween the inlet and the outlet for containing an aqueous solution ofurea and arranged such that, in use, the vessel and therefore the ureasolution become heated by means of heat exchange with the exhaust gas asit flows from the inlet to the outlet;c) a urea solution inlet to the reaction vessel and a gaseous hydrolysisproduct outlet from the reaction vessel;d) a pump for pumping urea solution into the reaction vessel via theurea solution inlet; ande) control means for controlling the pump in response to changing NOxoutput from the IC engine;wherein, in response to an increase in said NOx output, the controlmeans controls the pump to increase the level of urea solution in thereactor vessel, thereby increasing the surface area of urea solutionavailable for heat exchange with the exhaust gas so as to increase therate of production of gaseous hydrolysis product in the reactor vessel.

Preferably, in response to a decrease in said NOx output, the controlmeans controls the pump to decrease the level of urea solution in thereactor vessel, thereby decreasing the surface area of urea solutionavailable for heat exchange with the exhaust gas so as to decrease therate of production of gaseous hydrolysis product in the reactor vessel.

Preferably, the device further comprises a sensor placed within theexhaust gas flow to measure the quantity of NOx therein.

Advantageously, the NOx sensor may be upstream or downstream of the SCRcatalyst and would either measure the NOx output of the engine or theNOx output of the vehicle respectively. If the NOx output of the engineis measured then the signal is used to predict the required volume ofthe gaseous hydrolysis product required to be dosed into the gas toeffect its removal (i.e. open loop control), whereas if the NOx outputof the vehicle is sensed then more or less gaseous hydrolysis productwill be dosed into the exhaust gas depending whether the sensed NOxlevel is above or below a target level (i.e. closed loop control)

Alternatively, engine management data, for example torque, engine speed,and/or throttle setting, are interrogated in order to deduce the NOxoutput of the vehicle.

Preferably, the device includes a reservoir for receiving and storinggaseous hydrolysis product. More preferably, the device includes aconduit for interconnecting the reservoir and the exhaust system. Mostpreferably, the conduit includes valve means to selectively control thefeed of hydrolysis product stored in the reservoir into the exhaust gasvia the conduit.

Preferably, level and/or temperature and/or pressure sensors areprovided in the reactor.

Preferably, all the sensors required in the reactor are provided in asingle cluster, removable in its entirety to minimise the number ofaccess points required in the pressurised reactor. Preferably there isadditionally a quality sensor provided in the reservoir and optionallyin the urea storage tank to monitor the quality (for example theconcentration) of the urea. Preferably the level sensor also acts as thequality sensor.

Preferably the device is provided with ammonia sensors downstream of theSCR catalyst to measure the ammonia slip. Preferably temperature sensorsare provided inside the SCR catalyst to measure the temperature of thecatalyst. Preferably there are also sensors provided upstream and/ordownstream of the SCR catalyst to fully measure the temperature changesof the exhaust gas as it passes through the catalyst.

Preferably, the device includes a valve in the outlet from the reactionvessel, the valve being adapted to cause the contents of the reactionvessel, in use, to attain an elevated pressure as it becomes heated, andto discharge gaseous hydrolysis product into the reservoir

The valve may take a number of forms. In one preferred arrangement thevalve actuates in response to the pressure within the reactor. This canbe an active actuation where the pressure is measured in the reactor andthe valve is actuated via a control system depending on the signalreceived from a pressure transducer situated in the reactor.Alternatively this can be a passive actuation where the valve is selfactuating when a preset pressure occurs on its inlet side, i.e. it is asimple mechanical back pressure valve. In an alternative preferredarrangement the valve actuates in response to the temperature of theaqueous solution of urea. This is preferably done by measuring thetemperature within the aqueous urea solution and actuating the valve inresponse to the measured temperature. As the reaction occurs within thereaction vessel and the pressure rises the temperature within thesolution also rises until both are elevated, and as there is a directrelationship between the two, control of the release of the gaseoushydrolysis product can be based on either. The valve for controlling therelease of the gaseous hydrolysis product is preferably placed in thebulkhead between the reactor and the reservoir.

In a preferred arrangement the device further includes an auxiliaryheating means for heating the reservoir, thereby enabling the reservoirto become heated prior to the engine being started, or alternativelyenabling the reservoir to be maintained at an elevated temperature whenthe engine is switched off. The auxiliary heating means is preferably anelectrically powered heater or a diesel burning heater.

Preferably the device further comprises a bypass valve which canselectively control the proportion of the exhaust gas which is inthermal contact with the reactor to control the heat input into it.

Preferably the device is adapted for use with mobile, for examplevehicle, engines. As the hydrolysis reaction favours fairly stableconditions then in such applications, and due to the transient operatingconditions, it necessary to have a reservoir to store some of thehydrolysis product so the system can respond quickly to changes in therequirement for said hydrolysis product. This results in a residualvolume of hot, pressurised, ammonia containing, hydrolysis gas in thereservoir when the engine is shut down. The content of the hydrolysisgas will depend on the reagent which is initially used which may forexample be aqueous urea or ammonium carbamate. Both these reagents and anumber of others will result in a hydrolysis gas containing steam andcarbon dioxide as well as the ammonia. As the reservoir cools below 60degrees, ammonia and carbon dioxide will react to form ammoniumcarbamate which will then at least partially dissolve in the water whichforms as the steam condenses. Preferably the reservoir acts as asecondary reactor to, when the engine is re-started, heat the contentstherein to evaporate the water and decompose the ammonium carbamate intothe carbon dioxide and ammonia from which it formed.

Preferably there is a holding area into which, in response to a desireto reduce the liquid volume within the reactor, an amount of the aqueousurea is moved for temporary holding. Preferably the holding area isseparate from the aqueous urea storage tank.

Preferably when it is required to increase the liquid-volume within thereactor, if there is any liquid in the holding area, the reactor isfilled from the holding area until it is empty upon which, if furtherfilling is required, the reactor will be filled from the aqueous ureastorage tank.

Preferably the holding area is maintained at a temperature above whichsolids form within the liquid.

Preferably, both the reactor and the reservoir are heated by heatexchange with the exhaust gas.

Preferably, the device includes a catalyst arranged within the reactorto advance the rate of hydrolysis of the aqueous solution. Morepreferably the catalyst is arranged on a substrate. Most preferably thesubstrate is conical or frustoconical.

According to a fifth aspect of the present invention there is provided amethod of controlling the generation of a gaseous hydrolysis productcomprising ammonia, and the feeding of that product into the exhaust gasof an IC engine, the method comprising the steps of:

a) providing a housing having an inlet for the exhaust gas and an outletfor the exhaust gas;b) providing a reaction vessel located at least partially within thehousing between the inlet and the outlet for containing an aqueoussolution of urea and arranged such that, in use, the vessel andtherefore the urea solution become heated by means of heat exchange withthe exhaust gas as it flows from the inlet to the outlet;c) providing a urea solution inlet to the reaction vessel and a gaseoushydrolysis product outlet from the reaction vessel;d) providing a pump for pumping urea solution from into the reactionvessel via the urea solution inlet;the method further comprising the steps of:e) hydrolysing an aqueous solution of urea (as hereinbefore defined) atelevated temperature and pressure within the reactor vessel;j) determining the level of NOx in the exhaust gas;g) controlling the pump to increase the level of urea solution in thereactor vessel in response to an increase in NOx levels in the exhaustsystem, thereby increasing the surface area of urea solution availablefor heat exchange with the exhaust gas so as to increase the rate ofhydrolysis in the reactor vessel.

According to a sixth aspect of the present invention there is provided athermo-hydrolysis reactor for producing ammonia-containing gas byheating an aqueous solution of urea (as hereinbefore defined), thereactor comprising an elongate vessel having a middle tubular section,an enlarged lower section having an inlet therein for the solution, andan enlarged upper section having an having an outlet therein for theammonia-containing gas, said reactor being adapted such that, in use,heat transmitted through the walls of the reactor from an external heatsource heats the solution therein causing it to hydrolyse producing saidammonia-containing gas.

The reactor is designed for use with liquid reagents which hydrolyse toform ammonia-containing gas; in particular the reactor is designed foruse with aqueous solutions containing urea.

Preferably, in use, the thermo-hydrolysis reactor is heated by heatexchange with the hot exhaust gasses of an internal combustion engine.

Preferably the level of the aqueous solution of urea in the reactor isvariable and the reactor is configured such that, as the level of theaqueous solution of urea in the reactor increases, the wetted surfacearea to volume ratio of the reactor also increases.

In a preferred arrangement the enlarged lower section has conical sidesand the ratio of the maximum diameter of the lower conical section tothe diameter of the tubular section, and the angle of the sides of thelower conical section, define the relationship between fill level andwetted surface area of the reactor.

Preferably the reactor is provided with a level sensor to detect thelevel of the reagent within the reactor. In one arrangement the levelsensor passes through the lower end of the reactor and extendssubstantially vertically upwards into it, thereby maintaining themajority of the sensor substantially at the temperature of the liquidwithin the reactor. Alternatively the level sensor passes through theupper end of the reactor and extends substantially vertically downwardsinto it.

Preferably, situated within the reactor below the level of the outletand above the level of the solution is a baffle to prevent splashes ofaqueous urea from entering the ammonia-containing gas outlet.

Preferably a catalyst is placed in the reactor vessel to promote thehydrolysis of the aqueous solution of urea. More preferably the catalystextends from below the level of the aqueous solution of urea within thereactor to above the level of the aqueous solution of urea therebyenabling the contact area of the catalyst to be varied by changing thevolume of aqueous solution of urea within said reactor

Additionally the reactor may have a plurality of heat exchange fins onits exterior and/or interior. In one preferred arrangement the heatexchange fins placed on the interior of the reactor are made of ahydrolysis catalyst.

Preferably the reactor is provided with a supplementary heater suchthat, if necessary, the reactor may be heated by both heat exchange withthe exhaust gas and the supplementary heater. Preferably the reactor isprovided with temperature and pressure sensors to sense the temperatureand pressure within the reactor.

According to the a seventh aspect of the present invention there is alsoprovided a NOx-reduction system including a reactor as defined above anda road vehicle containing such a system.

According to an eighth aspect of the present invention there is providedan apparatus for generating an ammonia-containing gas for use in theselective catalytic reduction of NOx contained in the exhaust gases ofan IC engine, the apparatus comprising:

a) a hydrolysis reactor for containing an aqueous solution of urea (ashereinbefore defined)b) means for heating the solution to an elevated temperature by way ofheat exchange with said exhaust gases, whereby the urea is hydrolysedand the ammonia containing gases are liberated;c) valve means operable between a substantially closed position forenabling the pressure of the ammonia-containing gas to attain apredetermined elevated pressure within the reactor, and an open positionwhen the gas is above said predetermined pressure;d) a reservoir having an inlet for receiving all of theammonia-containing gas discharged from the reactor when said valve is inits open position, and an outlet for feeding ammonia-containing gas tothe exhaust gases, the reservoir serving to store: ammonia-containinggas during operation of the IC engine and, following the IC engine beingswitched off, ammonia-containing gas condensate; ande) means for heating the reservoir,the arrangement being such that on cold start-up of the IC engine, themeans for heating the reservoir is operable to decompose the condensateinto ammonia-containing gas.

By decomposing the condensate into ammonia-containing gas a sourcethereof is thereby provided at cold start-up of the IC engine for use inNOx reduction before the hydrolysis reactor reaches its normal workingelevated temperature and pressure.

Preferably, during normal operation the reservoir is maintained at apressure above the pressure within the exhaust conduit.

The reservoir, by providing a store of ammonia containing gas duringnormal operation of the system enables a fast response to transientchanges in the demand for ammonia to be dosed as the load on the enginechanges. While it is relatively easy to use a system without a reservoirand where the ammonia is effectively produced “on demand” in a situationwhere there is little or only gradual changes in the demand on thesystem, in a highly dynamic operating situation such as that foundonboard a commercial or a passenger vehicle there will normally be atime lag between a change in engine operating conditions and theammonia-containing gas supply being matched to those conditions due tothe finite time taken to hydrolyse the reagent “on demand”. By placingthe reservoir between the point of generation of ammonia-containing gasproduct and point of introduction to the exhaust, the requirement forammonia containing-gas to be dosed into the exhaust can be substantiallymet in real time. In addition the separation of the reservoir from thereactor ensures that the operating conditions within the reactor arekept constant, i.e. the pressure within the reactor does not fluctuateas a result of dosing the ammonia-containing gas into the exhaust,therefore resulting in a substantially consistent gas product mixtureexiting the reactor.

Preferably the reactor is solely heated by thermal heat transfer withthe exhaust gas effecting a simple heating system utilising the “free”energy available in the exhaust. To that end, the reactor is preferablyplaced within the exhaust conduit such that there is direct contactbetween the hot exhaust gas and at least a part of the exterior surfaceof the reactor.

Alternatively the reactor may be heated at least in part by electricmeans. Preferably the reactor is initially heated by both heat exchangewith the exhaust conduit and electric means and, once the exhaustreactor is at operating temperature and pressure, the electric heatingmeans is turned off and the reactor is maintained at operatingtemperature and pressure by heat exchange with the exhaust gas only.

In another alternative preferred arrangement the reactor is preheated byelectric heating means prior to the IC engine being started such thatthe reactor can produce ammonia-containing gas substantially immediatelyfrom the time the IC engine is started.

When the IC engine is turned off there is a residual volume ofammonia-containing gas within the reservoir and the ammonia-containinggas will continue to be produced for short time. As the reservoir coolsthe pressure of the ammonia-containing gas in the reservoir drops andthe H₂O condenses on the surface of the reservoir. As the temperatureand pressure further drop some of the ammonia and carbon dioxide willcombine to produce ammonium carbamate which then dissolves in thecondensed water forming a solution of ammonium carbamate. The pressureand temperature within the reactor will also drop and the gas productcontained within the reactor will undergo a similar process, the aqueousammonium carbamate mixing with the aqueous urea solution within thereactor. As the ammonia-containing gas product within the reservoircools and condenses, the pressure within the reactor will drop tosubstantially atmospheric pressure, preferably slightly belowatmospheric pressure, as will the pressure within the reactor, therebysubstantially eliminating the danger of ammonia escaping form the systemwhile the engine is not running. This is particularly important formobile IC engines, for example commercial or passenger vehicles wherethe vehicle may be stored within an enclosed space, for example a garagewhere any ammonia escaping from a pressurised system would be in acontained environment creating a build up of contained ammonia.

During cold start, i.e. when the IC engine is started from ambienttemperature there is a time period before the reactor will produceammonia-containing gas product for use in the NOx reduction processresulting in a time lag before ammonia is available for use in the NOxreduction process. This time lag is the result of a combination ofseveral factors including: the time taken for the exhaust gas to reachits normal operating temperature (compounded by the fact that IC enginesare normally started under no load or very light load conditionstherefore taking longer to reach normal operating temperatures), thecoefficient of thermal transfer between the exhaust gas and the liquidreagent within the reactor and the ratio of volume of liquid within thereactor to head space above the liquid. The result is that meetingrequirements of emission standards is difficult during initial start up.As emissions standards are becoming ever increasingly stringent, thisperiod during which the NOx is untreated will become unacceptable.

On cold start, heat is applied to the reservoir which then acts as asecondary reactor, evaporating the condensed water and thermallydecomposing the ammonium carbamate dissolved therein to create ammoniaand carbon dioxide gas thus reverting the contents of the reservoir backto their original state prior to the IC engine being shut down. Whenoperational the reservoir is maintained at an elevated temperature toprevent the gasses therein condensing. Preferably the reservoir ismaintained at a substantially constant temperature.

Preferably, heat is supplied to the reservoir by heat transfer from thehot exhaust gas both during normal operation and cold start-up. For thatpurpose the reservoir is preferably located such that a part of itprotrudes through, or forms a part of, the exhaust conduit. As the heatand time required for the cold-start reaction in the reservoir is muchless than that needed to drive the hydrolysis reaction in the reactor,the gas from the reservoir can be made available much sooner forintroduction to the exhaust.

However, preferably, an electric heating element is provided forinitially heating the contents of the reservoir which may be used inisolation or in combination with the heat supplied by the hot exhaustgasses.

In another preferred arrangement the electric heater is used on start upto supplement the heat transfer from the hot exhaust gas thus enabling afaster reaction of the aqueous ammonium carbamate. Preferably once thesystem is up to operational temperature the electric heating element isnot used and the temperature of the reservoir is substantiallymaintained by the exhaust gas. In periods of low engine load when theexhaust gas is relatively cool the electric heater may be used tosupplement the heating effect of the hot exhaust gas. When an electricheater is used during old start the heater is preferably turned onbefore the IC engine is started such that the aqueous ammonium carbamatewithin the reservoir is substantially thermally decomposed intoammonia-containing gas such that it is immediately available on start upof the engine.

In one preferred arrangement the reservoir is isolated from directcontact with the hot exhaust gas by an air gap, optionally containing aninsulating material, and is provided with an electric heating element.Heat transfer across the air gap is sufficient to produce the majorityof the heat needed to maintain the reservoir at an elevated temperatureunder operating conditions and the electric heater is used in start upand, if needed, to supplement the heating effect of the heat transferwith the exhaust gas.

Preferably the reservoir has a means of losing heat to the environmentsuch that a balance of heat input to heat output can be achievedapproximately at its operating temperature such that continued input ofheat does not cause the reservoir to continue to rise.

In an alternative arrangement where it is preferable to control thetemperature of the reservoir independently of the exhaust gastemperature, the reservoir is placed completely externally of theexhaust conduit and preferably is heated by means of its proximity tothe exhaust conduit. Preferably an electric heater is provided for useon start up to thermally decompose the aqueous ammonium carbamate asdescribed above. The electric heater, or an additional heater, mayoptionally also heat the entire outer surface of the reservoir to ensureno re-condensation of the gas occurs during start up. Preferably, oncethe system is up to operational temperature the electric heatingelement(s) is not used and heat input to the reservoir is provided byradiated and conducted heat from the exhaust gas. Where more accuratecontrol of the temperature of the reservoir is required a variablecooling circuit is provided operable to remove excess heat and maintainthe reservoir at a substantially constant temperature less than thetemperature of the exhaust gas. Preferably this cooling circuit iseither a part of the engine cooling circuit or has a heat exchanger totransfer heat to the engine cooling or lubrication circuit. Preferablythe reservoir is maintained at a temperature between 125 and 250 degreescentigrade, more preferably between 180 and 225 degrees centigrade.

In one preferred arrangement the reservoir is substantially positionedexternally from the exhaust conduit but has a section that extends intothe exhaust conduit for heat transfer arranged such that any liquidwithin the reservoir drains toward the section extending into theexhaust conduit. On start up any liquid within this section is directlyacted on by the hot exhaust gas converting it to ammonia-containing gas.Preferably the part of the reservoir extending into the exhaust conduitcomprises a heat pipe.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings in which;

FIG. 1 is a perspective view of the device in accordance with theinvention;

FIG. 2 is a longitudinal cross section through the device of FIG. 1;

FIG. 3 is a transverse cross section through a first reaction vesseldesign of the device of FIG. 1;

FIG. 4 is a schematic representation of a control system including thedevice of FIG. 1;

FIG. 5 is a transverse cross section through a second reaction vesseldesign of the device of FIG. 1;

FIG. 6 is a longitudinal cross section through the device of FIG. 1incorporated with an SCR catalyst;

FIG. 7 is a longitudinal cross section through the device of FIG. 1incorporated within a housing, the housing containing the device, aparticulate removal device and an SCR catalyst;

FIG. 8 is a perspective view of a second design of the device withvariable heating;

FIG. 9 is a transverse cross section of the device of FIG. 8;

FIG. 10 is a perspective view of the device of FIG. 1 including a dieselparticulate filter;

FIG. 11 is a perspective view of a third design of device according tothe invention;

FIG. 12 is a perspective view of the rear of the device of FIG. 11;

FIG. 13 is a perspective view of the device shown in FIG. 11 with theouter cover removed;

FIG. 14 is a cross section through the reservoir and reaction vessel ofFIG. 11;

FIG. 15 is a perspective view of a reservoir upper manifold according tothe present invention;

FIG. 16 is an exploded assembly drawing of the reservoir upper manifoldof FIG. 15 and its associated components;

FIG. 17 is a perspective view of the assembled reservoir upper manifoldof FIG. 15 and its associated components;

FIG. 18 is a schematic representation of a control system according tothe present invention.

FIG. 19 is a cross section of a reactor according to the invention;

FIG. 20 is a cross section of a reactor of the invention with heatexchange fins;

FIG. 21 is an cross section of an alternative reactor of the inventionwith a supplementary heater; and

FIG. 22 is a cross section of another reactor of the invention.

FIG. 23 is an embodiment of a system according to the invention;

FIG. 24 is an alternative embodiment of the invention incorporating aheat pipe;

FIG. 25 is an alternative embodiment of the invention incorporatingelectric heating;

FIG. 26 is an alternative embodiment of the invention incorporatingelectric heating and a heat pipe;

FIG. 27 is an alternative embodiment of the invention incorporating acooling system;

FIG. 28 is a vertical cross section showing a reservoir adjacent theexhaust conduit; and

FIG. 29 is a horizontal cross section through the arrangement shown inFIG. 28.

Referring to FIGS. 1 to 4 a device 1 is shown capable of being placedin-line in the exhaust conduit of an IC engine, for example that foundon a diesel vehicle, upstream of an SCR catalyst. The device 1 producesa gaseous product which is added to the exhaust gas in a controlledmanner to pass therewith through the SCR catalyst to reduce the NOxcontent of the exhaust gas. The device 1 has an inlet 2 and an outlet 3for the exhaust gas flowing therethrough and comprises an outer body 4,which forms a pressure barrier, and passing through the outer body 4 isan inner body 5 which comprises a flowpath longitudinally therethroughfrom the inlet 2 to the outlet 3. The device 1 is split into twosections, the first section is for hydrolysing an aqueous solution ofurea at elevated temperature and pressure so that it decomposes to forma gaseous hydrolysis product containing ammonia gas. In the firstsection the outer 4 and inner 5 bodies form two chambers 6, 7therebetween, substantially above the inner body and substantially belowthe inner body respectively. The lower chamber 7 has an inlet 8 forreceiving a supply of aqueous urea solution delivered by a pump 32(shown in FIG. 4 only) The upper chamber 6 has an outlet 9 for gaseoushydrolysis product. The lower 7 and upper 6 chambers are connected by aplurality of tubular elements 10 which pass through the inner body 5 andwhich form fluid flowpaths between the lower 7 and upper 6 chambers. Theupper 6 and lower 7 chambers and the tubular elements 10 together formand enclosed reaction vessel in which the hydrolysis reaction occurs.

In use the aqueous solution of urea is fed into the reaction vessel viathe inlet 8 in the lower chamber 7 by the pump 32. The level of aqueousurea in the reaction vessel is measured by a level sensor 11. Althoughthe level sensor 11 is shown to be only in the upper chamber 6, forgreater control over the liquid level within the reaction vessel it mayextend into the lower chamber 7 through one of the passageways oralternatively a second level sensor 12 may be placed in the lowerchamber (FIGS. 3 and 4).

The exhaust gas from the engine, which has a temperature up to around550 degrees centigrade (dependent on engine load), passes over the tubes10 and the upper and lower surfaces of the lower 7 and upper 6 chambersrespectively, raising the temperature of the liquid contained therein byheat exchange. As the temperature rises the hydrolysis reaction startsto occur (at approximately 60 degrees centigrade) and the gaseoushydrolysis product starts to collect in the headspace above the liquidlevel in the upper chamber 6. As the temperature rises further thereaction accelerates and a head of pressure builds up in the head space,pressurising the reaction vessel and allowing the temperature of theaqueous urea solution to rise above the temperature at which it wouldotherwise boil. The reaction vessel outlet 9 in the upper chamber 6includes a valve 13 which opens passively at a predetermined setpressure, preferably in the region of 15 to 20 bar, ideally 17 bar. Thusthe pressure in the reaction vessel is elevated above atmosphericpressure but is maintained below a certain value (in this case 17 bar),which gives a good reaction rate without the need to contain excessivepressures.

Alternatively the valve 13 may be active, i.e. it may operate inresponse to a pressure sensor 14 within the header section of thereservoir 15 (as will be described in further detail shortly).

The valve 13 releases the excess pressure from the reaction vessel intothe second section of the device which comprises a reservoir 15 whichsurrounds the inner body 5. The passage of hot exhaust gas through theinner body 5 heats the reservoir and keeps the ammonia containinghydrolysis product in its gaseous state. The reaction vessel has anoutlet 16 and a dosing valve 17 associated therewith. The device isfurther provided with a pressure sensor 18 to sense the pressure in thereservoir 15.

Referring now to FIG. 4, the pump 32 delivers aqueous urea solution froma holding tank 35 into the lower chamber 7 via the inlet 8. The pump 32is controlled by a controller 33 which is also connected electrically tothe level sensors 11, 12; reaction vessel outlet valve 13; dosing valve17; reaction vessel pressure sensors 14, 18; and an engine managementsystem 34 as indicated by the dashed lines in FIG. 4. The enginemanagement system 34 logs and controls the performance characteristicsof the IC engine in known manner.

The device 1 is operable as follows. The controller 33 receives a supplyof data from the engine management system 34, the data including, forexample, engine speed, torque, ignition timing and throttle position.This data is used to calculate the NOx level in the engine exhaustaccording to known techniques, such as executing algorithms on theengine management data or referencing look up tables. Given the NOxlevel in the exhaust, the controller 33 then calculates the volume ofammonia gas required to react with the prevailing level of NOxestablished in the exhaust.

Accordingly, in times of increased engine demand, for example highengine speed and/or torque, the controller 33 controls the pump 32 toincrease the rate of delivery of aqueous solution into the reactionvessel. This results in an increase in the level of aqueous solutionwithin the reaction vessel. Thus, a greater surface area of the insideof the reaction vessel becomes wetted by the aqueous solution. Theresulting increase in the heated wetted area in the reactor vessel, iethe total surface area of aqueous solution directly exposed to heat fromthe exhaust, causes increased heat transfer from the exhaust gas to theaqueous solution. This in turn generates an increased rate of productionof gaseous hydrolysis product.

In this manner, the controller 33 delivers an increased volume ofaqueous solution into the reaction vessel in response to an increase inthe level of NOx in the exhaust gas.

Conversely, in times of decreased engine demand, the controller 33controls the pump 32 to decrease the rate of delivery of aqueoussolution into the reaction vessel. This results in a reduced rate ofproduction of gaseous hydrolysis product.

In times of increased load, the NOx levels in the exhaust increase. Thisincreases the demand for ammonia gas, in response to which thecontroller 33 controls the pump 32 to increase the rate of delivery ofaqueous solution to the reaction vessel. However, increased engine loadalso delivers an increase in exhaust gas temperature and flow rate. Inparticular, a high engine speed will lead to a high exhaust gas flowrate and high torque operation will increase the exhaust gastemperature. Consequently, in times of high load, an increase in theheated, wetted area of aqueous solution in the reaction vessel isobserved concurrently with an increased exhaust gas temperature and/orflow rate. Accordingly not only is the rate of production of gaseoushydrolysis product increased by virtue of the increased heated wettedarea, but also by the increased rate of heat transfer per unit areadelivered by the increase in exhaust gas temperature and/or flow rate.In this way the increased volume of aqueous solution in the reactorvessel is balanced by the increased rate of gaseous hydrolysis product.This leads to a reduction in the volume of aqueous solution and therebya stabilisation in the level of aqueous product within the reactionvessel.

However, there exist engine operating conditions where the increasedrate of delivery of aqueous solution is not matched by an increasedexhaust gas temperature and/or pressure. For example, where an engine isunder high load driving the vehicle up a steep incline, NOx levels inthe exhaust will increase. However, upon reaching the end of the inclinethe engine may operate at tick over or very low load, for example innegotiating a decline. The exhaust temperature will consequentlydecrease leading to a mismatch in exhaust conditions and rate of aqueoussolution delivery into the reaction vessel. Under such circumstances,the controller 33 controls the pump 32 to pump aqueous solution from thereaction vessel into a holding reservoir (not shown for clarity). Thesolution is held in the holding vessel until such time as the demand forammonia increases at which point the controller 33 controls the pump 32to pump the solution from the holding vessel into the reaction vessel.The holding vessel may be heated by an auxiliary heating means in orderto prevent condensation of the gaseous hydrolysis product. The holdingvessel is evacuated before the pump pumps aqueous solution from the tankin order to retrieve the heat retained in the solution in the holdingvessel by virtue of its earlier heating in the reaction vessel.

In the event that engine load rapidly increases following a period oflow load, for example beginning a steep incline having previouslycompleted a low load descent, the second level sensor 11 is provided toensure that the level of aqueous solution in the reaction vessel doesnot become dangerously low.

As discussed above the gaseous hydrolysis product is released into thereservoir 15 when the headspace pressure in the reaction vessel risesabove 17 bar. The dosing of the gas from the reservoir is controlled asfollows. The valve 17 is operable in response to a signal from thecontroller 33 to open and allow some of the gas within the reservoir 15to enter the exhaust gas flowing through the inner body 5 to flowtherewith through an SCR catalyst (not shown) positioned downstream ofthe device. The controller 33 monitors the reservoir pressure viapressure sensor 18 and calculates the required opening of the valve (forthe given pressure) to introduce the required volume of hydrolysisproduct (or component thereof) dictated by the engine exhaustconditions. Optionally the reservoir temperature is also monitored aswill be discussed in further detail shortly.

Accordingly, the reservoir 15 acts as a buffer between the reactionvessel and the IC engine exhaust. The reservoir depletes and replenishesso as to allow for the lag in the control of the rate of production ofgaseous hydrolysis product in response to the prevailing exhaustconditions.

Referring to FIG. 5 an alternative arrangement of the reaction vesselsection is shown in which the inner body 5 sits within the outer body 4and is arranged such that there is an upper chamber 6 and a lowerchamber 7 substantially above and below the inner body 5, and apassageway 19 formed between the walls of the inner 5 and outer 4chambers making them in fluid communication. The reaction vessel acts insubstantially the same manner as described in reference to FIGS. 1 to 4and the fluid is heated as it passes from the lower to the upper chamberby conduction through the walls of the inner body 5. Optionally acombination of the two designs may be utilised whereby the fluid passesfrom the lower chamber 7 to the upper chamber 8 through the tubes 10(see FIG. 3) and through the passageways 19.

Referring to FIG. 6 the device as described in relation to FIGS. 1 to 4is shown incorporated with an SCR Catalyst 20. The SCR catalyst 20 iscontained within an open ended housing 21 which fits around the outerbody 4 of the device. It may be attached by any means known in the art,for example a simple screw thread or bayonet type fitting. The outlet 3of the device feeds directly into the SCR catalyst, the exhaust gassespassing therethrough and exiting the extended form of the device via SCRoutlet 22. In an alternative arrangement (not shown) the outer wall 4 ofthe device extends beyond the reservoir 15 and in its extended regionforms a housing for the SCR Catalyst.

Referring to FIG. 7 the device as described in relation to FIGS. 1 to 4is shown incorporated within a common housing with the SCR catalyst anda particulate removal device. The common housing 23 has an inlet 24 andan outlet 25 and located therebetween a particulate removal device 26,the device of the invention 27 and the SCR catalyst 28. The particulateremoval device may be any such device known in the art, for example adiesel particulate filter.

Referring to FIGS. 8 and 9 the device as described in FIGS. 1 to 4 isshown which additionally comprises a heat exchange bypass. The innerbody 5 is split in two longitudinally by means of a dividing plate 29 tocreate a heat exchange section containing the tubes 10 and a bypasssection 30, and a diverter flap 31 is placed upstream of dividing plate29. The diverter flap 30 is movable by controller 100 (see FIG. 4) toselectively allow a varying amount of the exhaust gas to flow over thetubes 10 thereby controlling the heat input into the reaction vessel.This enables the speed at which the hydrolysis occurs, and therefore thespeed at which the ammonia-containing gas is produced, to be controlledby way of the position of the diverter plate in addition to, or insteadof, the rate of delivery of aqueous solution to the reaction vessel.

Referring to FIG. 10 a gas treatment apparatus is shown particularlyuseful for treating the exhaust gas of a commercial vehicle engine. Anouter housing comprising endplates 41, 42 is split into three sectionsby plates 43 and 44 forming two end sections 45, 46 and a centralsection 47. An exhaust inlet 48 passes through plate 41 and section 45and opens into an oxidation catalyst 49 situated in central section 47and extending between plates 44 and 45. The outlet of the oxidationcatalyst passes through plate 44 opening into end section 46, theexhaust gas expanding as it does so. Also located in the central section47 between plates 43 and 44 are two SCR catalysts 50, 51 their inletsbeing in end section 46 and their outlets discharging into end section45 such that the exhaust gas entering end section 46 then passes throughthe SCR catalysts into end section 45. Situated in plate 44 and leadingfrom end section 45 is an inlet into a closed end baffle drum 52 whichhas a number of outlets 53 in the side of the drum opening into centralsection 47. Also located in central section 47 is a second closed endbaffle drum 54 which has a plurality of inlets 55 in the side of thedrum 54 and an outlet 56 leading from the drum 54 and passing throughend section 46 and out of the apparatus for discharge to atmosphere.Located within end section 46 is a hydrolysis reaction vessel 57 asdescribed in relation to FIG. 10 which has an inlet 58 for a pressurisedsupply of urea and an outlet 59 for ammonia containing gas. The reactionvessel 57 is heated by heat exchange with the exhaust gas circulatingwithin the end section 46 as it passes from the outlets of the oxidationcatalyst 49 to the inlets of the SCR catalysts 50, 51. Also locatedwithin end section 46, but separated from the gas flow therein by abaffle plate 60, is a valve unit 61 containing a pressure control valvefor controlling the pressure within the reaction vessel and a dosingvalve for controlling the flow of ammonia containing gas to twoinjection points 62. Each injection point 62 injects ammonia containinggas into the exhaust gas stream prior to it passing through the two SCRcatalysts 50, 51 wherein the ammonia reacts with the NOx in the exhaustas on the surface of the SCR catalysts 50, 51 reducing the NOx contentof the exhaust to a level acceptable for discharge to atmosphere viaoutlet 56. The pressure control valve of the valve unit 61 has an outletleading a gas reservoir 63 which provides a buffer of ammonia containinggas ready to be dosed into the exhaust gas via the dosing control valveof the valve unit 61 and the injection points 62. The gas reservoir 63is situated in the central section 47 of the apparatus in which theexhaust gas passing from baffle drum 52 to baffle drum 54 is circulatingand is thereby heated by heat exchange with the hot exhaust gas.

Referring to FIGS. 11 to 14, an alternative embodiment of gas treatmentdevice 64 is shown which operates in a substantially similar manner tothe embodiment described previously. The exhaust gas of an IC engineflows through the device 64 from an inlet 65 to an outlet 66. Theexhaust enters the inlet 65 containing NOx and leaves the outlet 66substantially free on NOx. The device 64 may be attached to a commercialor passenger vehicle and connected in line in the existing vehicleexhaust system.

When the exhaust gas passes through the inlet 65 it passes a NOx sensor112 before entering a first cylindrical tube 67 containing a hydrolysisreaction vessel 68 (see FIG. 14). The hot exhaust gasses exit the tube67 through an opening therein and enter an enclosed cavity 69. As thehot exhaust gasses pass over the reaction vessel 68 it absorbs heat fromthe gasses and becomes elevated in temperature. The reaction vessel 68has an inlet 70 at its lower end through which an aqueous solution ofurea is supplied. The aqueous solution is delivered from a holding tank110 by a pump 11, both shown schematically in FIG. 14 only.

As the reaction vessel 68 becomes heated the aqueous solution of ureastarts to hydrolyse and hydrolysis gasses form in the head space abovethe level of the urea. The reaction vessel 68 is provided with apressure relief valve 71 in its upper end which allows the hydrolysisgas to pass from the reaction vessel 68 to a reservoir 72 if thepressure in the reaction vessel 68 exceeds 17 bar.

The tube 67 has a closed upper end (with an opening therein throughwhich the pressure relief valve 71 projects). The reaction vessel 68 isattached to the device by its upper end.

The enclosed cavity 69 has a passageway in one of its walls (not shown)allowing the exhaust gas to exit the cavity 69 and pass through anoxidation catalyst 74 where a percentage of the NO in the exhaust gas isoxidised into NO₂. The exhaust gas then exits the oxidation catalyst andenters a truncated conical section 75 which reduces in diameter.

A feed tube 76 leads from the reservoir into the conical section 75 andthe hydrolysis gas is dosed through the feed tube 76 into the exhaustgas at the open end of the cone. As the flow reduces mixing is inducedbetween the exhaust gas and the hydrolysis gas. After the conicalsection 75 the exhaust gasses pass around a 90° bend 82 and flows into acylindrical vortex mixer 83. The exhaust gasses enter the vortex mixer83 tangentially and exit along its central axis into an SCR catalyst 84wherein the hydrolysis gas mixes with the NOx converting itsubstantially to nitrogen and water. The exhaust gas exits the SCRcatalyst 84 and expands into the interior of the device enclosed bycover 85. The treated exhaust gasses then exit the device via the outlet66 which passes through the enclosed cavity 69. Arranged in proximity tothe exit 66 are a NOx sensor 113 and an ammonia sensor 114.

The flow of hydrolysis gas from the reservoir 72 into the conicalsection 75 via the tube 76 is controlled by a dosing valve 77 (as willbe described in further detail shortly) attached to an upper manifold 78of the reservoir 72. The reservoir 72 is located in a tube 79 andpositioned such that there is an air gap between the reservoir 72 andthe tube 79. Part of the outer surface of the tube 79 forms a wall ofthe enclosed cavity 69 and as such is in direct contact with the hotexhaust gasses. In use the reservoir becomes heated by heat transferfrom the exhaust gas through the tube 79 and across the air gap. Thereservoir 72 is elongate in shape and similar to the reaction vessel 68will expand in length. The reservoir 72 is attached at its upper end andfree to expand at its lower end. A sliding seal 80 is provided to retainthe lower end of the reservoir 72. A heater 81 is situated at the lowerend of the reservoir to allow for additional heating to supplement theheat from the exhaust gasses. The pressure release valve 71 and thedosing valve 77 are maintained in a cooler area and are separated fromthe warmer area by a manifold plate 86, which may either be of athermally shielding material or may include a thermal shield. Thepressure relief valve 71 and the dosing valve 77 have covers 87, 88sealed thereover maintaining them in a clean and dry environment.

Referring to FIGS. 15 to 17, a reservoir upper manifold 89 for use in agas reservoir containing ammonia and carbon dioxide for use in an SCRprocess is shown. It can be used, for example, as the reservoir in thesystem described with reference to FIGS. 12 to 14. The upper manifoldforms the upper end of the reservoir 72 (see FIG. 14) and is welded tothe reservoir body 90. The upper manifold 89 has a plurality ofpassageways in it adapted to accommodate associated components.Passageway 91 is the gas inlet to the reservoir and is fed with asupplied of ammonia containing gas via tube 92. Passageway 93 is theinlet for a dosing valve 94. The gas from the reservoir enters the valve94 through port 93 and exits through pipe 95 which passes back into thereservoir and passes through the reservoir body 90 via a bulkheadfitting (not shown). The dosing valve 94 is operable to control the flowof ammonia containing gas from the reservoir into the exhaust gas flowof an IC engine. Passageway 96 accommodates a safety valve 97 whichopens above a preset pressure and vents excess gas pressure out of thepassageway 96, past the safety valve 97 and sideways out throughpassageway 98 from where it flows through a tube 99 into the exhaust gasflow.

Passageway 100 accommodates a reservoir pressure sensor 101. Passageway102 accommodates a fitting 104 to accept a reservoir temperature sensor103 which detects the temperature of the gas within the reservoir. Thesame sensor (or a second sensor) can also monitor the temperature of theupper manifold itself.

The upper manifold 89 has a plurality of ports 105 in its sides toaccommodate heating elements 106. If the ammonia and carbon dioxidegasses cool down in the presence of each other then they can form solidsalts, e.g. ammonium carbamate, which can block the valves resulting innot only the inability to dose the gas into the exhaust gas but also apossibly dangerous increase in pressure within the reservoir.Alternatively a build up of solids may occur on the sensors 101, 103causing them to malfunction, again possibly leading to a dangerousincrease in pressure within the reservoir. The heaters 106 are operatedto maintain the upper manifold 89 at a raised temperature to preventsolidification of any salts in any of the passageways therethrough. Theheaters maintain the upper manifold 89 above 130° C., ideally at asubstantially constant temperature of 220° C. Between the upper manifoldand the components is a thermal barrier 107 to protect the componentsfrom heat radiated directly from the upper manifold. The upper manifoldhas a number of threaded holes 108 therein for attaching a cover 109 toit. The thermal barrier also acts as a gasket and seals the cover 109over to the manifold, thus the reservoir can be washed, for example witha powerful spray of water, without water ingress into the associatedcomponents 92, 94, 101, 104 and any associated electronics. The cover109 is made of aluminium has a plurality of cooling fins to assist inrapid heat loss from this section maintaining the components withintheir working temperature range.

In use the device 64 is operable as follows. A controller (not shown inFIGS. 11 to 17 for clarity) is provided which is electrically connectedto the dosing valve 77, reservoir pressure sensor 101, reservoirtemperature sensor 103, heaters 81, 106, NOx sensors 112, 113 andammonia sensor 114.

The device 64 may optionally be provided with an analogue level sensorfor measuring the exact level of aqueous solution in the reaction vessel68, the level sensor also being connected electrically to thecontroller. The reaction vessel may also have optional temperature andpressure sensors for communicating to the controller the reaction vesselconditions in order to control an active pressure release valve in placeof the passive unit described above.

In distinction to the first embodiment, the controller of the secondembodiment receives a signal from the NOx sensor 112 rather thancalculating the exhaust NOx levels by derivation from engine load data.

The volume of ammonia gas required to react with the NOx level detectedin the exhaust gas is calculated and the pump 111 controlled accordinglyto increase or decrease the rate of flow of aqueous solution into thereactor. The level of aqueous solution in the reaction vessel rises orlowers accordingly, thereby altering the rate of heat transfer betweenthe aqueous solution and the exhaust gas as described previously.

The controller also monitors downstream NOx levels in the exhaust by wayof NOx sensor 113 in order to ensure that NOx consumption is maximised.Like wise the controller monitors ammonia levels in the exhaust gasexiting the device 64 by way of an ammonia sensor 114 in order tominimise the risk of ammonia slip.

The controller also monitors the reservoir temperature and pressure byway of temperature sensor 103 and pressure sensor 101. When thereservoir temperature and/or pressure fall below predetermined values,the reservoir heater 81 is operated to raise the reservoir temperaturein order to prevent the gaseous hydrolysis product condensing.

This is particularly advantageous at cold start-up of the IC engine asthe residual condensate in the reservoir is heated to provide ammoniafor delivery into the exhaust before the exhaust gas has raised thetemperature of the reactor sufficiently to cause hydrolysis of theaqueous solution.

In a further embodiment the reaction vessel includes a conical catalystsubstrate. The varying cross-sectional area of the substrate with heightfurther emphasises the effect of altering the rate of hydrolysis bychanging the level of aqueous solution in the reaction vessel.Alternatively, the substrate may have a form other than conical, forexample cylindrical in order to deliver a particular change in reactionrate per unit increase in the liquid height.

Referring now to FIG. 18, a control methodology 200 is shown forcontrolling the devices 1, 64 described above. The methodology isdescribed hereafter with reference to the gas treatment device 64 but isequally applicable to the gas treatment device 1. A demand generator 202receives a catalyst condition signal 204 from the exhaust catalyst, aNOx sensor signal 206 from the NOx sensor 112, an engine conditionsignal 208, from the engine management system (not shown for clarity)and a demand signal 210.

The demand generator calculates a required ammonia output rate anddelivers an ammonia output signal 212. The ammonia output signal isdelivered to a dosing valve control 214 and a pump control 216. Thedosing valve control 214 outputs a dosing valve signal 218 to commandthe opening and closing of the dosing valve 77. In order to calculatethe required dosing valve signal 218, the dosing valve control 214receives a reservoir pressure signal 220 and a reservoir temperaturesignal 222 from the reservoir pressure sensor 101 and reservoirtemperature sensor 103. The reservoir pressure signal 220 is alsodelivered to the pump control 216 in addition to an integral anddifferential of the pressure signal. The pump control 216 outputs a pumpsignal 224 to control the pump 111. In order to generate the pump signal224, the pump control 216 may optionally also receive a reactor levelsignal 226 from a reactor level sensor (not shown for clarity). Thereservoir temperature signal 222 is also delivered to a reservoir heatercontrol 228 which generates a reservoir heater signal to control thereservoir heater 81.

Optionally, a reactor pressure sensor delivers a reactor pressure signal232 to a reactor pressure control 234 which outputs a pressure reliefvalve signal 236 to an active pressure relief valve for venting gaseoushydrolysis product from the reactor into the reservoir. This optionalcontrol methodology is only required when an active pressure reliefvalve is used in place of a passive valve.

It will be appreciated that various components and control methods aredescribed in respect of one or other of the embodiments. Nonetheless anyof the measurement and control features described above areinterchangeable between embodiments.

Referring to FIG. 19 a thermo-hydrolysis reactor 1901 is shown, capableof being placed in-line in the exhaust conduit of an IC engine, forexample that found on a diesel vehicle, upstream of a selectivecatalytic reduction (SCR) catalyst. The thermo-hydrolysis reactor mayfor example be used in the SCR system described with reference to FIGS.11 to 18. The thermo-hydrolysis reactor 1901 produces anammonia-containing gaseous product which is added to the exhaust gas ina controlled manner to pass therewith through an SCR catalyst to reducethe NOx content of the exhaust gas. The reactor 1901 comprises anelongate body 1902 with enlarged upper 1903 and lower 1904 sections. Thereactor 1901 is provided with an inlet 1905 for the supply of aqueousurea solution and an outlet 1906 for the removal of theammonia-containing gas. The release of the ammonia-containing gas viathe outlet 1906 is controlled by a pressure control valve in the outletline (not shown). Entering the reactor 1901 from the top is a levelsensor 1907, the output of which is used to control a pump (not shown)supplying inlet 1905 to maintain the urea liquid level 1908 betweenlower 1909 and upper 1910 liquid level measurement points. Also enteringthe top of the reactor are a pressure 1911 and temperature 1912 sensor.In use, the reactor 1901 is heated by heat transfer with hot exhaustgas. The aqueous solution of urea becomes heated and starts to decomposeforming hydrolysis gasses comprising ammonia, carbon dioxide and steam.As the hydrolysis gases collect in the upper section 1903 of the reactorthey are prevented from leaving by the pressure control valve in theoutlet line and thus the pressure in the reactor increases to the setpressure of the control valve. The increase in pressure allows for afurther increase in temperature, the increased temperature and pressureresulting in a shortened-hydrolysis time. Eventually the pressure in thereactor 1901 exceeds the set pressure of the pressure control valvewhereby “excess” ammonia-containing gas issues from the outlet 1906 viathe control valve for use in the SCR process. A reactor of this designis particularly appropriate for use in a mobile application, for exampleon board commercial vehicle as, due to its tall, thin geometry, theliquid level in the reactor will remain substantially unaffected by suchfactors as the vehicle being on an incline, centrifugal force of thevehicle following a radial path or the reagent sloshing due to unevenmotion of the vehicle. All the sensors 1907, 1911, 1912 comprise asingle sub assembly which is attached to the reactor at one end, therebygiving a single access point enabling simple replacement should any ofthe sensors fail.

Referring to FIG. 20 a reactor 2013 for use in an exhaust gas treatmentapparatus, for example for use in the SCR system described withreference to FIGS. 11 to 18, is shown comprising an elongate body 2014with a bulbous head section 2015 and a conical lower section 2016.During use the reactor 2013 is heated by heat transfer from the hotexhaust gasses of an engine (not shown) to hydrolyse the aqueous ureatherein. The reactor 2013 has a level sensor 2017 entering at its topand extending downwards therefrom into the aqueous urea within thereactor 2013. The liquid level sensor 2017 is situated on the centralaxis of the reactor 2013. By placing the liquid level sensor 2017 on thecentral axis as the liquid moves slightly from side to side the level atthe central axis should not change significantly. Preferably the liquidlevel sensor 2017 measures the liquid level 2018 on a continuous scale.The reactor 2013 has an inlet 2019 for the supply of pressurised aqueousurea and an outlet 2020 which leads to a pressure control valve (notshown). The reactor 2013 has a baffle 2021 situated in its head section2015 above the liquid level and below the outlet 2020. In the event ofany splashing of the reagent within the reactor 2013, for example due tomotion of the vehicle the baffle 2021 prevents splashes of liquid fromexiting from the outlet 2020. The liquid level 2018 may be controlled bycontrolling the volume of aqueous urea pumped into the reactor via inlet2019 dependant on the sensed liquid level. The heat transfer from thehot exhaust gas is dependent on the wetted surface area of the reactor2013. The geometry of the conical section 2016 allows for a specific nonlinear relationship of heat transfer to liquid level to be achieved. Toassist heat transfer from the exhaust gas to the reactor 2013 a numberof heat exchange fins 2022 are shown on the external surface of thereactor 2013. The surface area of the fins 2022 changes in relation tothe height of the reactor 2013 and thus the heat input to the aqueousurea can be controlled by varying the liquid level 2018. For additionalheat transfer to the liquid heat exchange fins 2023 fins are showninside the reactor 2013 to increase the contact surface area between thereactor body 2014 and the aqueous urea within the reactor 2013. Thereactor 2013 is also provided with temperature 2024 and pressure 2025sensors to monitor the temperature and pressure of the gas within thereactor 2013.

Referring to FIG. 21 a reactor 2126 for use in an exhaust gas treatmentapparatus, for example for use in the SCR system described withreference to FIGS. 11 to 18, is shown comprising an elongate body 2127with a bulbous head section 2128 and a conical lower section 2129.During use the reactor 2126 is heated by heat transfer from the hotexhaust gasses of an engine (not shown) to hydrolyse the aqueoussolution of urea therein. The reactor has a level sensor 2130 enteringat its top and extending downwards therefrom into the aqueous solutionof urea within the reactor. The reactor 2126 has an inlet 2131 in thelower section 2129 and an outlet 2132 in the upper section 2128, saidinlet 2131 and outlet 2132 comprising bulkhead fittings 2133, 2134 forattaching the reactor to a bulkhead 2135 which may for example be theexhaust conduit. The lower section 2129 of the reactor 2126 contains asupplementary heating element 2136 which is situated below the liquidlevel 2137, said liquid level 2137 being maintained within a rangedetected by the liquid level sensor 2130. The supplementary heater 2136is used during start up to enhance the heating capacity of the hotexhaust gas to decrease the time taken for the reactor 2126 to reach itsoperating conditions of temperature and pressure measured by temperatureand pressure sensors 2138, 2139. Outlet 2132 leads to a pressurecontroller which, in use, maintains an elevated pressure within thereservoir 2126. A hydrolysis catalyst 2140, for example tungstenvanadium, is provided within the reactor below the level 2137 of theurea solution. Alternatively (not shown) the catalyst may extend frombelow the liquid level to above the liquid level whereby variation ofthe liquid level exposes the aqueous urea to a greater or a lessersurface area of the catalyst.

Referring to FIG. 22 a reactor 2241 for use in an exhaust gas treatmentapparatus, for example for use in the SCR system described withreference to FIGS. 11 to 18, is shown having an enlarged upper section2242 and lower section 2243. The reactor 2241 contains an aqueoussolution of urea up to a level 2244 detected by level sensor 2245 whichextends upwards from the bottom of the reactor 2241. The reactor has anaqueous urea inlet 2246 in its lower section for supplying the reactorwith a supply of aqueous urea which in use, becomes heated by means ofheat exchange with hot exhaust gas through the walls of the reactor2241. The reactor 2241 is attached at its upper end to the exhaustconduit 2247 and a pressure regulating valve 2248, situated outside theconduit 2247 is in communication with the interior of the reactor 2241through the conduit 2247. The valve 2248 has an outlet 2249 throughwhich the ammonia containing hydrolysis gas passes for use in SCR of NOxin exhaust gasses. The reactor 2241 has a slosh baffle 2250 to helpprevent splashes of the aqueous solution from entering the valve via thereactor outlet 2251.

Referring to FIG. 23 a system of the invention is shown which comprisesa reactor 2301 fed through an inlet 2302 with a supply of pressurisedaqueous urea solution. The urea is approximately 32% urea by volume,ideally AdBlue available from GreenChem Holdings B.V. The rate of supplyis regulated by a pump 2303 which is controlled in response to a liquidlevel indicator (not shown) situated within the reactor 1301 to maintainthe reactor 2301 in a partially full condition. The reactor 2301 issituated within the exhaust conduit 2304 of an IC engine such that theflow of hot exhaust gas passes over the reactor 2301 heating the ureatherein. As the temperature rises the urea starts to break down byhydrolysis producing ammonia-containing gas, thus raising the pressurein the head space in the reactor 2301 above the liquid level. A pressurecontrol valve 2305 is situated towards the top of the reactor 2301 andonce the pressure within the reactor 2301 reaches a set pressure,preferably about twenty bar, any excess gas produced passes through thepressure control valve 2305 thereby maintaining the pressure within thereactor 2301 substantially constant. The temperature is also maintainedsubstantially constant giving substantially constant operatingconditions for the hydrolysis process. After passing through thepressure control valve the ammonia-containing gas enters a reservoir2306 which is situated partially within, and partially outside of, theexhaust conduit 2304. The reservoir 2306 has one section within theexhaust conduit 2304 which is heated by the exhaust gas passing over itwhich prevents the ammonia-containing gas from condensing orcrystallising during normal operation of the engine, and has a secondsection external to the flow of the exhaust gasses which allows for heatloss from the reservoir 2306 such its temperature is lower than that ofthe reactor 2301. The ammonia-containing gas is however still maintainedat an elevated temperature and at a pressure above those of the exhaustgasses. A valve 2307 is controlled to release gas from the reservoir2306 into the exhaust gas flowing through the conduit 2304 via nozzle2308. The ammonia-containing gas then passes with the exhaust gassesthrough an SCR catalyst (not shown) where it reacts with the NOx in theexhaust gas on the surface of the SCR catalyst resulting in reduced NOxemissions. When the IC engine is shut down and therefore the exhaustgasses stop flowing, it loses heat to its environment and the systemwill gradually cool down. As it does so the hydrolysis process will stopand the ammonia-containing gas within the reservoir 2306 will start tocondense, eventually forming a pool of aqueous solution of ammoniumcarbamate (which may also contain a small amount of ammonia and carbondioxide) in the base of the reservoir. As the condensation occurs thepressure within the reservoir 2306 will drop eventually reaching apressure which is approximately atmospheric pressure or slightly below.When the IC engine is restarted the hot exhaust gas will start to flowover the reactor 2301 and the reservoir 106 thereby heating them.However the reactor 2301 will take a finite amount of time to reach itsoperating pressure and temperature before it can produce moreammonia-containing gas. In the interim, the pool of aqueous ammoniumcarbamate in reservoir 2306 will be thermally decomposed and revert backinto its previous gaseous form and be available for use in a shortertime than the new ammonia-containing gas produced by the reactor 2301.This allows for ammonia containing gas to be applied to the hot exhaustgas for SCR sooner after start-up of the engine, reducing the NOxemissions in the initial period prior to the reactor producingammonia-containing gas.

Referring to FIG. 24 another embodiment of the invention is shown inwhich a reactor 2401 is fed in the same way as in FIG. 23 by conduit2402 and pump 2403. In this embodiment the reactor 2401 ends at backpressure valve 2405 located adjacent to, but externally of, the exhaustconduit 2404. The majority of the reservoir 2406 is situated externallyof the exhaust conduit 2404 and has a valve 2407 for controlling theflow of the ammonia-containing gas from the reservoir 2406 into theexhaust conduit 2404 via a nozzle 2408 to mix with the hot exhaust gaspassing therein. The reservoir includes a small heat pipe 2409 whichextends through the exhaust conduit 2404 and is in direct contact withthe exhaust gas. The general operation of the system is as described inreference to FIG. 1. The reservoir 2401 is shaped such that when the ICengine is shut down and the cooling of the system condenses theammonia-containing gas, the condensate will collect in the heat pipe2409. On start up, therefore, the condensate is all in contact with thehot exhaust gas. In addition, a supplementary electric heater 2410 isprovided such that additional heat can be put into the condensate toaccelerate its re-conversion back to gaseous form, thereby reducing theNOx emissions by further reducing the time lag between start up of theIC engine and having ammonia-containing gas ready for addition to thesystem for use in SCR. During normal running of the system, once it isup to temperature the electric heater 2410 is turned off, the reservoirbeing maintained at an elevated temperature by heat transfer conductbetween the hot exhaust gasses and the part of the reservoir 2406 withinthe exhaust conduit 2404.

Referring to FIG. 25 another embodiment of the system is shown in whichthe reservoir 2501, conduit 2502, pump 2503, and pressure control valve2505 operate in the same manner as their corresponding parts in FIG. 24.The reservoir 2506 is situated completely externally form the exhaustconduit 2504 and as such is not directly heated by the hot exhaustgasses. The reservoir is joined to the exhaust conduit via valve 2507and nozzle 2508 to allow the ammonia-containing gas as within thereservoir to be applied to the exhaust gas prior to them flowingtogether through an SCR catalyst (not shown). The reservoir 2506 isheated by an electric heater 2511 which raises the temperature of thereservoir 2506 to, and maintains it at, at a temperature above which thegasses therein will start to condense. The heater is controlled tomaintain the reservoir at a substantially constant temperature in theregion of 200 degrees centigrade.

Referring to FIG. 26 a system is shown which is a combination of FIGS.24 and 25, and the components work in the same manner. The reservoir2606 is provided with a small heat pipe 2609 situated at the bottom ofthe reservoir 2606 but external to the exhaust conduit 2604. When the ICengine is turned of and the ammonia-containing gas within the reservoir2606 condenses, the resulting solution will collect in the heat pipe2609. The heat pipe is provided with an electric heater 2610 which is ofa high power to quickly reconvert the solution to ammonia-containing gasready for dosing. The reservoir 2606 is also provided with a generalheater 2611, which may be of lower power for the general heating of thereservoir 2606.

Referring to FIG. 27 a system of the invention is shown which comprisesa reactor 2701 fed by an inlet 2702 with a supply of pressurised aqueousurea. The flow of supply is regulated by a pump 2703 which is controlledin response to a liquid level indicator 2712 situated within the reactor2701 to maintain the reactor 2701 in a partially full condition. Thereactor 2701 is situated within the exhaust conduit 2704 of an IC enginesuch that the flow of hot exhaust gas passes over the reactor 2701heating the urea therein hydrolysing it to produce reaction gasses whichare a mixture of ammonia, H₂O and CO₂. A pressure control valve 2705 issituated towards the top of the reactor 2701 and once the pressurewithin the reactor 2701 reaches a set pressure, preferably about twentybar, any excess gas produced passes through the pressure control valve2705 thereby maintaining the pressure within the reactor 2701substantially constant. After passing through the pressure control valvethe ammonia-containing gas enters a reservoir 2706 which is situatedpartially within, and partially outside of, the exhaust conduit 2704.The reservoir 2706 has one section within the exhaust conduit 2704 whichis heated by the exhaust gas passing over it which prevents theammonia-containing gas from condensing or crystallising, and has asecond section external to the flow of the exhaust gasses which allowsfor heat loss from the reservoir 2706 such that its operatingtemperature will be lower than that of the reactor 2701. As thereservoir 2706 may be in an environment which has an elevatedtemperature, the natural temperature loss through the reservoir 2706 toits environment may not be sufficient, and there is no possibility tocontrol the final temperature as it will be dependent on ambienttemperature. Therefore the reservoir 2706 is surrounded by a coolingcoil 2713 which is pumped by a variable speed pump 2714 through a heatexchanger 2715. The heat exchanger 2715 is in turn cooled by heatexchange with the cooling system of the IC engine which typicallymaintains a fairly constant temperature. The speed of pump 2714 can becontrolled to maintain a substantially constant temperature within thereservoir 2706. A valve 2707 is controlled to release ammonia-containinggasses from the reservoir 2706 into the exhaust gas flowing through theconduit 2704 via nozzle 2708. The ammonia-containing gas then passeswith the exhaust gasses through an SCR catalyst (not shown) where theyconvert the NOx in the exhaust. When the IC engine is shut down andtherefore the exhaust gasses stop flowing, as it loses heat to itsenvironment, the system will gradually cool down. As it does so thehydrolysis process will stop and the ammonia-containing gas within thereservoir 2706 will start to condense, eventually forming a pool ofaqueous solution of ammonium carbamate (which may also contain a smallamount of ammonia and carbon dioxide) in the base of the reservoir. Whenthe IC engine is restarted the hot exhaust gas will start to flow overthe reactor 2701 and the reservoir 2706 heating them up. The pump 2714will not start to circulate the cooling fluid within the coil 2713 untilthe reservoir reaches its operating parameters. The reservoir 2701 willfunction as described above and will take a finite amount of time toreach its operating pressure and temperature before it can produce moreammonia-containing gas. In the interim, the pool of aqueous ammoniumcarbamate within the reactor 2706 will be thermally decomposed andrevert back into its previous gaseous form and be available for use in ashorter time than the new ammonia-containing gas being produced by thereactor 2701. This allows for the ammonia containing gas to be appliedto the hot exhaust gas sooner to start up of the engine, reducing theNOx emissions in the initial period prior to the reactor producingammonia-containing gas. An auxiliary heater 2716 in the reactor 2701 canbe used during start up to supplement the heat from the exhaust gassesto decrease the time taken for the reactor to reach operatingparameters.

Referring to FIGS. 26 and 27, the introduction of the heater 2716 ofFIG. 27 into the system of FIG. 26 would enable a system wherein priorto the starting of the IC engine, heaters 2716, 2609 and 2611 could bepowered to bring the system up to operating temperature such that it isready to apply ammonia-containing gas to the exhaust gasses as soon asthe IC engine is started, thereby eliminating any delay between thestarting of the engine, and therefore the production of NOx, and itsreduction by the system of the invention.

Referring to FIG. 28 a section view of the reservoir 2806 of a system isshown located in a chamber 2817 adjacent the exhaust conduit 2804. Anair gap 2818 separates the reservoir 2806 from the conduit and heattransfer across the air gap 618 heats the reservoir 2806. The rate ofheat transfer across this gap may be controlled by adding an insulationmaterial in the air gap 2818. The reservoir 2806 has an inlet and outletwith associated dosing valve (not shown). The reservoir 2806 has aheating element 2811 associated therewith. The heater 2811 can be usedprior to, or during, start up to heat the condensate in the reservoir2806 and revert it to its gaseous state ready for dosing into theexhaust gas flowing through the conduit 2804.

Referring to FIG. 29 a top view of FIG. 28 is shown. The reservoir 2906is located in a chamber 2917 adjacent the exhaust conduit 2904 and hasan air gap 2918 surrounding it. A first part 2919 of the surface of thechamber 2917 forms is in contact with the hot exhaust gasses and theremainder of the surface chamber 2917 is exposed to the atmosphere andheat is lost through that part. Preferably the ratio of the surface areaof the first part 2919 to the remainder of the surface is such that atsome operating conditions an equilibrium of heat input to heat lost isachieved so that the reservoir 2906 is maintained at a substantiallyconstant temperature.

1. A unitary device for generating and feeding gaseous hydrolysisproduct comprising ammonia, formed by the hydrolysis of an aqueoussolution of urea (as hereinbefore defined) at elevated temperature andpressure, into the exhaust gas of an IC engine as it flows through theexhaust system of the engine to an SCR catalyst, the device beingadapted to be placed in the exhaust system so that the exhaust gas willflow through it during use, and comprising a) a housing having an inletfor the exhaust gas and an outlet for the exhaust gas; b) a reactionvessel located at least partially within the housing between the inletand the outlet for containing an aqueous solution of urea and arrangedsuch that, in use, the vessel and therefore the urea solution becomeheated by means of heat exchange with the exhaust gas as it flows fromthe inlet to the outlet; c) a urea solution inlet to the reaction vesseland a gaseous hydrolysis product outlet from the reaction vessel; d) areservoir for receiving and storing gaseous hydrolysis product; e) avalve in the outlet from the reaction vessel and adapted to permit thecontents of the reaction vessel, in use, to attain an elevated pressureas it becomes heated, and periodically to discharge gaseous hydrolysisproduct into the reservoir; and f) a conduit for interconnecting thereservoir and the exhaust system, the conduit including a valve toselectively control the feed of hydrolysis product stored in thereservoir into the exhaust gas via the conduit.
 2. The device accordingto claim 1 wherein the valve in the outlet is placed at least partiallyoutside the housing such that it is at least partially protected fromdirect exposure to the hot exhaust gasses
 3. The device according toclaim 1 wherein the conduit for interconnecting the reservoir and theexhaust system is placed at least partially outside the housing suchthat it is at least partially protected from direct exposure to the hotexhaust gasses
 4. The device according to claim 1, wherein the valve inthe outlet actuates in response to the pressure within the reactionvessel.
 5. The device according to claim 4 wherein the valve in theoutlet is a mechanical back pressure valve and allows excess gas to passthrough once the pressure within the reaction vessel exceeds a specificpredetermined pressure.
 6. The device according to claim 4 wherein thevalve in the outlet is actuated via a control system in response to asignal received from a pressure transducer situated in the reactionvessel indicating the pressure therein is above a specific value.
 7. Thedevice according to claim 1, wherein the valve in the outlet is actuatedvia a control system in response to a signal received from a temperaturesensor situated in the reaction vessel indicating the temperature of theaqueous solution of urea therein is above a specific value.
 8. Thedevice according to claim 1, further comprising an SCR catalyst withinthe unitary device.
 9. The device according to claim 8 wherein thedownstream end of the SCR catalyst is coated with a catalyst thatconverts any un-reacted ammonia in the exhaust gas into harmless gassessuch that ammonia is not released into the environment.
 10. The deviceaccording to claim 1, wherein the gaseous hydrolysis product isintroduced substantially on the axis of the exhaust gas flow pathsubstantially perpendicularly to the direction of the exhaust gas flow.11. The device according to claim 10 wherein a number of radially spacedinlets are situated adjacent to one another substantiallyperpendicularly to the flow.
 12. The device according to claim 1,wherein the point of introduction of the gaseous hydrolysis product issubstantially at the mouth of a truncated conical section of theflowpath and the flow of exhaust gas and gaseous hydrolysis product intothe cone induces mixing.
 13. The device according to claim 1, whereinthe flowpath between the point of introduction of the gaseous hydrolysisproduct and the SCR catalyst has at least one substantially 90 degreebend therein causing turbulence in the flowpath.
 14. The deviceaccording to claim 1, further comprising a substantially cylindricalvortex chamber, upstream of the SCR catalyst, wherein the exhaust gasand gaseous hydrolysis product enter the vortex chamber substantiallyperpendicularly to its radius and exit the chamber along its centralaxis, the vortex within the chamber further inducing mixing of exhaustgas and gaseous hydrolysis product.
 15. The device according to claim 1,further comprising an oxidation catalyst within the single unit andthrough which the exhaust gas flows prior to introduction of the gaseoushydrolysis product.
 16. The device according to claim 1, furthercomprising a diesel particulate filter and through which the exhaust gasflows prior to introduction of the gaseous hydrolysis product.
 17. Thedevice according to claim 1, further comprising: an oxidation catalystwithin the single unit and through which the exhaust gas flows prior tointroduction of the gaseous hydrolysis product; and a diesel particulatefilter and through which the exhaust gas flows prior to introduction ofthe gaseous hydrolysis product, wherein the diesel particulate filter isupstream of the oxidation catalyst
 18. A device according to claim 1,further comprising a NO_(x) sensor placed within the exhaust gas flow tomeasure the quantity of NOx therein.
 19. The device according to claim18 wherein the NOx sensor is placed in the exhaust flow upstream of thepoint of introduction of the gaseous hydrolysis product and the signalis used to predict the required volume of the gaseous hydrolysis productrequired to be dosed into the gas to effect its removal.
 20. The deviceaccording to claim 18 wherein the NOx sensor is placed in the exhaustflow downstream of the SCR catalyst and a greater or lesser amount ofgaseous hydrolysis product will be dosed into the exhaust gas dependingwhether the sensed NOx level is above or below a target level.
 21. Thedevice according to claim 1, further comprising an ammonia sensor placeddownstream of the SCR catalyst to measure ammonia slip.
 22. The deviceaccording to claim 1, wherein the device comprises an outer body, whichforms a pressure barrier, and passing through the outer body is an innerbody which comprises a flowpath longitudinally therethrough with aninlet and an outlet for the exhaust gas, the outer and inner bodiesforming two chambers therebetween, one substantially above the innerbody and one substantially below the inner body, the two chambersconnected by at least one fluid passageway.
 23. The device according toclaim 22 wherein the two chambers and the at least one fluid passagewaycomprise the reaction vessel.
 24. The device according to claim 23wherein the at least one fluid passageway is in thermal contact with theexhaust gas.
 25. The device according to claim 24 wherein the at leastone fluid passageway between the two chambers passes through the exhaustgas flowpath formed by the inner body.
 26. The device according to claim25 wherein the at least one fluid passageway passes around the sides ofthe inner body.
 27. The device according to claim 22, wherein the innerand outer bodies extend beyond the reaction vessel, the volume definedbetween said inner and outer bodies in their extended sections beingseparated from the reaction vessel at one end by a bulkhead and enclosedat the other end to form a reservoir area abutting the reaction vesseland through which the inner body passes.
 28. The device according toclaim 27 wherein the valve in the outlet is located in the bulkheadseparating the reaction vessel and the reservoir.
 29. The deviceaccording to claim 22, where a by-pass valve is provided to selectivelybypass a proportion of the exhaust gas so that it does not directly heatthe fluid passageways of the reaction vessel.
 30. The device accordingto claim 29 wherein the inner body comprises two exhaust gas flowpaths,only one of which is in thermal contact with the fluid passageways ofthe reaction vessel.
 31. The device according to claim 1, wherein thedevice comprises a rear section comprising two substantially cylindricalupright tubes and an enclosed cavity therebetween, said tubes adapted tohouse the reaction vessel and reservoir respectively.
 32. The deviceaccording to claim 31 wherein the tube housing the reaction vessel hasan inlet for the exhaust gas and is in fluid communication with theenclosed cavity such that the hot exhaust gasses flow in the inlet, overthe reaction vessel and exit the tube into the enclosed cavity.
 33. Thedevice as claimed in claim 31 wherein at least a part of the tubehousing the reservoir is in direct fluid contact with the hot exhaustgasses.
 34. The device as claimed in claim 33 wherein the reservoir isisolated from direct heat contact with the part of the tube in directfluid contact with the hot exhaust gasses by an air gap.
 35. The deviceas claimed in claim 34 wherein heat transfer through the wall of thetube containing the reservoir and across the air gap is sufficient tomaintain the reservoir at a high enough temperature to preventsolidification of salts out of the hydrolysis gas.
 36. The device asclaimed in claim 31, wherein the enclosed cavity has an opening thereinfor the exhaust gas to pass through prior to entering a dieselparticulate filter and/or an oxidation catalyst.
 37. The deviceaccording to claim 31, wherein the catalysts and at least one mixingmeans is attached to the exterior of the rear section via a framework.38. The device according to claim 37 further comprising an outer casingthat fits over the catalysts and mixing elements forming a treatmentenclosure.
 39. The device according to claim 38 wherein the outlet forthe exhaust gas passes from the treatment enclosure through the enclosedcavity in the rear section to allow the exhaust gas to exit the unit foreventual discharge.
 40. The device according to claim 31, wherein thereservoir and reaction vessel abut a manifold plate, said manifold plateproviding a barrier between a hot area below it and a cooler area aboveit.
 41. The device according to claim 40 wherein the valves and anysensors are placed at least partially in the cooler area such that theirelectronics and some other function critical parts can be protected fromdirect exposure to the hot environment.
 42. The device according toclaim 40 wherein the manifold plate includes a heat shield between thehot area and the cooler area.
 43. The device according to claim 41wherein the valves and any sensors have covers sealed thereover toprevent water ingress into the electronics.
 44. The device according toclaim 43 wherein said covers comprise a thermally conductive materialand include a number of cooling fins to assist in removing any heat fromthis area.
 45. The device according to claim 31, mounted on a commercialvehicle such that the rear section is closest the centre of the vehicleand treatment enclosure extends outwards therefrom such that, in eventof a collision, the treatment area forms a sacrificial ‘crumple zone’ toabsorb the energy of impact and protect the pressurised reaction vesseland reservoir from direct impact.
 46. The device according to claim 1,further provided with a heating element for the reservoir.
 47. Ahydrolysis gas reservoir for receiving ammonia containing gas from ahydrolysis reaction vessel, the reservoir comprising a body and amanifold, said manifold having passageways therein to accommodatevarious sensors and at least one valve and having heating meansassociated therewith to maintain said manifold at an elevatedtemperature.
 48. The reservoir according to claim 47 wherein the heatingmeans comprises one or more electric heating elements.
 49. The reservoiraccording to claim 48 wherein the heating means comprises a plurality offinger heaters inserted substantially radially into the manifold. 50.The reservoir according to claim 47, wherein the heating means isadapted to maintain the manifold at a temperature in the range 130 to300 degrees centigrade.
 51. The reservoir according to claim 50 theheating means is adapted maintain the manifold at a temperature in therange 210 to 230 degrees centigrade.
 52. The reservoir according toclaim 47, wherein attached to a passageway of the manifold is a pressurerelief valve that releases the hydrolysis product from the reservoirshould the pressure therein exceed a certain value.
 53. The reservoiraccording to claim 52 wherein any gas being released via the pressurerelief valve is released into a small reservoir of water to condense thegaseous hydrolysis product and prevent it being released directly intothe atmosphere.
 54. The reservoir according to claim 52 wherein any gasbeing released via the pressure relief valve is released directly intothe exhaust gas flow.
 55. The reservoir according to claim 47, whereinattached to a port of the manifold is a dosing valve for dosing theammonia-containing gas into an exhaust gas stream.
 56. The reservoiraccording to claim 47, wherein the manifold has a valve seat thereinbetween two of said passageways, one passageway leading from theinterior of the reservoir and forming a valve inlet and the otherpassageway exiting the side of the manifold forming a valve outlet. 57.The reservoir according to claim 56 wherein a valve actuator andassociated valve armature are connected to said manifold, the valveactuator operable to move the valve armature on and off the valve seatthereby allowing or preventing flow therethrough.
 58. The deviceaccording to claim 47, wherein the reservoir manifold has means forattaching it to a manifold plate such that the valves and sensorsprotrude through the manifold plate into the cool area above it.
 59. Thereservoir according to claim 58 wherein said means for attaching themanifold to the manifold plate comprise a plurality of flanges adaptedto take a screw or bolt.
 60. A system for the reduction of NOx in theexhaust gas of an IC engine comprising a reactor for producing ammonia,a reservoir to temporarily store ammonia a means of introducing ammoniato the exhaust gas and an SCR catalyst, the reservoir including a bodyand a manifold, said manifold having passageways therein to accommodatevarious sensors and at least one valve and having heating meansassociated therewith to maintain said manifold at an elevatedtemperature.
 61. A device for generating gaseous hydrolysis productcomprising ammonia, formed by the hydrolysis of an aqueous solution ofurea at elevated temperature and pressure, the device being adapted tobe placed in the exhaust system so that the exhaust gas will flowthrough it during use, and comprising a) a first substantially uprightand cylindrical tube enclosed at its upper end and open at its lower endand having an inlet and an outlet on its sides for the exhaust gas; b)an elongate reaction vessel located in the tube for containing anaqueous solution of urea and arranged such that, in use, the vessel andtherefore the urea solution become heated by means of heat exchange withthe exhaust gas as it flows from the inlet to the outlet; and c) a ureasolution inlet to the reaction vessel and a gaseous hydrolysis productoutlet from the reaction vessel; wherein said reaction vessel isattached to the upper enclosed end of the first tube and sealinglyengages with the first tube at its lower end preventing the exhaust gasfrom escaping out of the open lower end of the first tube.
 62. Thedevice according to claim 61 wherein the reaction vessel is providedwith a structurally weak point in its upper end that will rupture at alower pressure that the rest of the reaction vessel ensuring that, inthe case of excessive pressure build up in the reaction vessel, thestructurally weak point will rupture and the gas in the reaction vesselwill expand therethrough forcing the reaction vessel downwards.
 63. Thedevice according to claim 61 wherein the reaction vessel has acircumferential seal attached to the outer surface of its lower end andthe said seal slides in the tube as the reaction vessel expands andcontracts.
 64. The device according to claim 61 wherein the tube has acircumferential seal attached to the inner surface of its lower end andthe reaction vessel slides past the seal as it expands and contracts.65. The device according to claim 61, wherein the device furthercomprises a second substantially upright and cylindrical tube having anenclosed upper end and an open lower end, said second tube housing asubstantially elongate reservoir to collect the gaseous hydrolysisproduct produced in the reaction vessel, said reservoir being attachedto the upper enclosed end of the tube and sealingly engaging with thetube at its lower end.
 66. The device according to claim 65 wherein theexterior of the second tube is at least partially heated by the hotexhaust gasses.
 67. The device according to claim 65 wherein thereservoir is provided with a structurally weak point in its upper endthat will rupture at a lower pressure that the rest of the reservoirensuring that in the case of excessive pressure build up in thereservoir the structurally weak point will rupture and the gas in thereservoir will expand therethrough forcing the reservoir downwards. 68.The device according to claim 65, wherein reservoir has acircumferential seal attached to the outer surface of its lower end andthe said seal slides in the tube as the reservoir expands and contracts.69. The device according to claim 65, wherein the tube has acircumferential seal attached to the inner surface of its lower end andthe reservoir slides past the seal as it expands and contracts.
 70. Thedevice according to claim 65, wherein the first and second substantiallyupright tubes form the two substantially upright tubes.